(1) Field
The present invention relates to a method of signal multiplexing and a transmitter in a radio frequency transmission system. The present invention may be used in the system for time-division-multiplexing and transmitting a signal from a user equipment as one example of the transmitter to a radio frequency base station as one example of a receiver, for example.
(2) Description of Related Art
Transmission systems of the radio frequency communication includes the transmission system, which cyclically copies a part of a valid symbols and adds a CP (Cyclic Prefix) (also referred to as a guard interval (GI)) to the valid symbol in time domain for reducing intersymbol interference due to a delay wave.
An OFDM (Orthogonal Frequency Division Multiplexing) and a DFT-S OFDM (Discrete Fourier Transform-Spread OFDM) are representative ones of such transmission systems.
Among them, the DFT-S OFDM is single carrier transmission, so that this is excellent in PAPR (Peak to Average Power Ratio) characteristics and is capable of operating a power amplifier (PA) at an efficient operating point.
Therefore, the DFT-S OFDM is preferred as the transmission system of up-link (UL), which is in a direction from a user equipment (UE) to a base station (BS or eNodeB), and in a 3GPP (3rd Generation Partnership Project) E-UTRA (Evolved Universal Terrestrial Radio Access), application of an SC-FDMA (Single Carrier Frequency Division Multiple Access), which is an access system using the DFT-S OFDM to the UL communication is discussed (for example, refer to Non-Patent Document 1).
Also, in FIG. 2 of Non-Patent Document 2, a transmission format for multiplexing an ACK/NACK signal relative to a shared channel transmitted by down link (DL) and a CQI (Channel Quality Indicator) signal indicating a propagation channel quality of the DL measured at the receiver adjacent to the block into which a reference signal (RS) used for propagation channel training (channel estimation) is multiplexed is disclosed.
That is to say, in the transmission format, one slot is formed of seven blocks, the RS is multiplexed into the fourth block in the slot, and the ACK/NACK signal and the CQI signal are multiplexed at the ending of the third block and the beginning of the fifth block, which are adjacent to the RS, respectively.
Meanwhile, in the 3GPP LTE (Long Term Evolution), the rules about the EVM (Error Vector Magnitude) and SEM (Spectrum Emission Mask) representing the quality of the signal and about the adjacent channel leakage ratio (ACLR) are set (refer to Non-Patent Document 3).    [Non-Patent Document 1] 3GPP TS36.211 V8.0.0    [Non-Patent Document 2] 3GPP TSG-RAN WG1, R1-073572, “Control Signaling Location in Presence of Data in E-UTRA UL”, Samsung    [Non-Patent Document 3] 3GPP TS36.101 V.0.1.0
In the transmission system for transmitting by adding the CP to the valid symbol, the signal is discontinuous at the boundary of each symbol (OFDM symbol and SC-FDMA symbol) after adding a CP, so that a frequency spectrum is infinitely spread and the electrical power leaks out of the signal band (this is also referred to as adjacent-band radiation).
In order to suppress this, in the transmission system, there is a case in which the pulse shaping is performed such that the signal is moderately attenuated in the vicinity of the symbol boundary by multiplying a window function (time window) such as a Raised Cosine function to the signal (symbol) and by filtering with a band-limiting filter or the like.
However, when such pulse shaping is performed, there is a case in which a signal attenuating portion due to the pulse shaping is included in the symbol, and the signal attenuating portion due to the pulse shaping of the adjacent symbol is mixed as the intersymbol interference at the time of detection of the valid symbol by eliminating the CP on the receiver side. Therefore, the signal quality (reception characteristics) such as the EVM or the like of the signal multiplexed in the vicinity of the symbol boundary is relatively easily deteriorated as compared to the signal multiplexed into other portions.
Also, in the radio communication system, there is a case in which the transmission power of the transmitter changes due to transmission power control or the like. In such a case, when this is a moderate power change as indicated by a solid line in FIG. 22 relative to an ideal power change as indicated by a dotted line in FIG. 22, for example, the quality of the signal (for example, EVM) multiplexed in the vicinity of the timing of the power change also is relatively easily deteriorated as compared with the signal multiplexed into other timings.
However, the above-described conventional technology does not perform the multiplexing of the transmission symbols in view of the characteristics that the signal quality in the vicinity of the symbol boundary and the power change point is easily deteriorated as compared to other portions. For example, the Non-Patent Document 2 only tries to improve the reception quality of the ACK/NACK signal and the CQI signal by multiplexing the ACK/NACK signal and the CQI signal at the temporally nearest (adjacent) position of the RS used for the propagation channel training.